Tsuneo Iwasaki

Effects of cholinergic and monoaminergic antagonists and tranquilizers upon spatial memory in rats

To explore the pharmacological mechanisms of the spatial memory, performance on the radial arm maze was tested in rats under the following drugs, using a within-subject design; scopolamine (0.25 and 0.5 mg/kg), methylscopolamine (0.5 and 1 mg/kg), phentolamine (5 and 10 mg/kg), propranolol (10 and 20 mg/kg), chlorpromazine (1 and 2 mg/kg), and chlordiazepoxide (5 and 10 mg/kg). The number of correct choices was significantly decreased by scopolamine, while the other drugs, including methylscopolamine, showed no effects on the correct choices. Almost all drugs affected the running time. These findings indicate that the brain cholinergic system is involved in the spatial memory.

Perirhinal N-methyl-d-aspartate and muscarinic systems participate in object recognition in rats

To determine the possible involvement of N-methyl-d-aspartate (NMDA) and muscarinic activation of the perirhinal cortex in object recognition, an NMDA antagonist (d,l-2-amino-5-phosphonopentanoic acid (AP5)) and a muscarinic antagonist (scopolamine) were injected into the perirhinal cortex of rats. A high dose of AP5 (60 mM) and two doses of scopolamine (20 and 80 mM), but not a low dose of AP5 (30 mM) alone, significantly impaired discrimination between novel and familiar objects in a spontaneous object recognition task, which is one of the recognition memory tasks. These results suggest that activation of both NMDA and muscarinic receptors in the perirhinal cortex contributes to object recognition.

Effects of intrahippocampal AP5 treatment on radial-arm maze performance in rats.

The purpose of the present study was to investigate the possible involvement of hippocampal NMDA (N-methyl-d-aspartate) receptors in spatial memory in rats by means of intrahippocampal injection of an NMDA antagonist, AP5 (D,L-2-amino-5-phosphonopentanoic acid). In Expt. 1, spontaneous motor activity (locomotor activity and rearing) was measured in an open-field after bilateral AP5 injection into the hippocampus, and it was shown that AP5 did not affect general activities within the dosage used in the present study. In Expt. 2, radial-maze performance was observed after the intrahippocampal injection of AP5. Rats were required to consume all the pellets which were put on each of the eight arms of the maze without revisiting the arms. AP5 produced dose-dependent impairment on the choice performance in the radial-arm maze, but did not have any effect on their running time. These results suggest that hippocampal NMDA receptors are involved in spatial memory.

Ethanol-induced state-dependent learning is mediated by 5-hydroxytryptamine3 receptors but not byN-methyl-d-aspartate receptor complex

Involvement of N-methyl-D-aspartate (NMDA) receptor complex and 5-hydroxytryptamine3 (5-HT3) receptors in state-dependent learning (SDL) induced by ethanol (EtOH) was investigated in the step-through passive avoidance task in rats. Pre-training injections of EtOH or MK-801 reduced step-through latency in the test session conducted 24 h after the training session. Pre-test as well as pre-training injections of EtOH failed to reduce the latency, while pre-training and pre-test injections of MK-801 reduced the latency. These results show that EtOH but not MK-801 produces SDL. SDL induced by EtOH was blocked by ICS205-930 injected before either the training or test session. However, ICS205-930 failed to block SDL induced by diazepam and muscimol. These results suggest that NMDA receptor complex may not be involved in SDL, and that 5-HT3 receptors may contribute to SDL induced by EtOH but not by diazepam and muscimol.

Intrahippocampal D-cycloserine improves MK-801-induced memory deficits: radial-arm maze performance in rats.

In order to investigate whether strychnine-insensitive glycine sites coupled with hippocampal NMDA (N-methyl-d-aspartate) receptors are involved in spatial memory in rats, we examined the effects of intrahippocampal treatment of d-cycloserine (DCS), a glycine-site agonist, on spatial-memory deficits which were produced by an NMDA antagonist MK-801 (dizocilpine) on the radial-arm maze task. After the acquisition of this task, the radial-maze performance was tested under the combined treatments of intraperitoneal MK-801 or saline (SAL) and intrahippocampal DCS or SAL. The results showed that MK-801 impaired the performance, and that DCS improved the MK-801-induced performance impairment. These results suggest that glycine sites are involved in spatial memory through their modulatory action on hippocampal NMDA receptors.

Involvement of benzodiazepine/GABA-A receptor complex in ethanol-induced state-dependent learning in rats

State-dependent learning (SDL) induced by ethanol (EtOH) was investigated on the step-through passive avoidance task in rats. Pretraining injection of EtOH dose-dependently reduced step-through latency in the test session 24 h after the training. Injection of EtOH (1.0 g/kg) before both the training and test sessions, however, failed to reduce the latency. These results show that EtOH produces SDL. The failure of learning performance in SDL (dissociation in SDL) induced by EtOH was blocked by bicuculline, Ro15-4513 and picrotoxin injected before the training session. The success of learning performance in SDL (non-dissociation in SDL) induced by EtOH was also blocked by bicuculline, Ro15-4513 and picrotoxin injected before the test session. The antagonism of Ro15-4513 against EtOH was blocked by flumazenil. In the substitution test, pretest injection of EtOH produced non-dissociation in SDL in the both of pretraining diazepam-and muscimol-treated rats. On the other hand, neither pretest injection of diazepam nor muscimol produced non-dissociation in the pretraining EtOH-treated rats: asymmetrical cross-substitution between EtOH and diazepam and between EtOH and muscimol was observed. These results suggest that the EtOH-induced SDL is partially mediated by the benzodiazepine (BDZ)/GABA-A receptor complex.

Interaction between benzodiazepine and GABA-A receptors in state-dependent learning

State-dependent learning (SDL) induced by benzodiazepine (BDZ) and GABA-A agonists was investigated in the step-through passive avoidance task in rats. Pre-training injection of diazepam or muscimol dose-dependently reduced step-through latency in the test session conducted 24 hr after the training. Injection of either drug before both the training and test sessions, however, failed to reduce the latency. The results show that passive avoidance failures induced by pre-training injections of diazepam and muscimol are due to SDL. In contrast to diazepam and muscimol, baclofen induced no SDL. Diazepam and muscimol were found to substitute for each other in producing SDL. The failure of learning performance in SDL (dissociation in SDL) induced by diazepam was blocked by flumazenil and picrotoxin but not by bicuculline injected before the training session, whereas dissociation in SDL induced by muscimol was blocked by flumazenil, bicuculline and picrotoxin. On the other hand, the success of learning performance in SDL (non-dissociation in SDL) induced by diazepam was blocked by flumazenil, bicuculline and picrotoxin injected before the test session, whereas non-dissociation in SDL induced by muscimol was blocked by bicuculline and picrotoxin but not by flumazenil. These results demonstrate that 1) BDZ and GABA-A agonists produce a common drug state, but, 2) roles of each receptor in SDL might be different, i.e., BDZ receptors for dissociation in SDL and GABA-A receptors for non-dissociation in SDL, and 3) chloride ion channels are essential for the induction of SDL by BDZ and GABA-A agonists.

Repeated treatment with N-methyl-D-aspartate antagonists in neonatal, but not adult, rats causes long-term deficits of radial-arm maze learning

Brain glutamatergic system is involved in synaptic plasticity as a base for learning and neural development. This study investigated the effects of neonatal and adult chronic antagonism of N-methyl-d-aspartate (NMDA) receptors, a subtype of glutamate receptors, on learning and/or memory. Rats were trained in the radial-maze learning, which is known as a measure of spatial working memory capacities, in adulthood after neonatal or adult repeated treatment of MK-801 (dizocilpine; 5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine), a non-competitive antagonist, or neonatal repeated treatment of CGS 19755 (cis-4-phosphonomethyl-2-piperadine carboxilic acid), a competitive antagonist. Neonatal repeated treatment of MK-801 or CGS 19755 markedly impaired the radial-arm maze learning. In addition, the treatment altered activities differently in the radial-maze and in the open-field. On the other hand, adult repeated treatment with MK-801 affected neither the radial-maze learning nor activities. Results suggest that chronic blockade of NMDA receptors in a neonatal stage may produce long-lasting deteriorative effects on spatial working memory in adulthood.

NMDA and muscarinic blockade in the perirhinal cortex impairs object discrimination in rats.

To determine the possible involvement of NMDA and muscarinic activation of the perirhinal cortex in object discrimination, an NMDA antagonist, D,L-2-amino-5-phosphonopentanoic acid (AP5), and a muscarinic antagonist, scopolamine (SCP) were injected into the perirhinal cortex of rats. Each drug at the higher dose (AP5 60 mM, SCP 80 mM) significantly decreased correct choices on the retention test of object discrimination. SCP, but not AP5, also significantly increased response latency, but this increase was not necessarily related to the time spent for a choice. These results suggest that activation of both NMDA and muscarinic receptors contributes to object discrimination.

CORTICOSTERONE LEVELS DURING EXTINCTION OF RUNWAY RESPONSE IN RATS

This investigation examined whether the hypothalamic-pituitary-adrenocortical axis would activate during extinction of a straight runway response. Four groups of rats were trained to run in a straight runway for 3 (for 3S and 3L groups) or 10 (for 10S and 10L groups) food pellets as a reward for either 15 (for 3S and 10S groups) or 25 (for 3L and 10L groups) acquisition days followed by 3 extinction days. Blood samples for determination of serum corticosterone concentration were taken from all animals under 5 different situations: 10 min after entering the experimental room, after the 24th acquisition day, after the first and third extinction days and under the deprivation control. Running speed of 10S and 10L groups in extinction days were reduced more quickly than that of 3S and 3L groups. After the third extinction day, serum corticosterone concentrations were significantly elevated in 10S and 10L, but not in 3S and 3L groups. These results indicate that the amount of reward in the acquisition phase influences not only the running speed, but also the corticosterone concentration in the extinction phase.